Category Archive now supports category slugs and titles

For example, in the YAML header set categories like this

categories:
 - awesome [Awesome Posts]
 - news

Octopress will parse out '[Awesome Posts]' and 'news' as the title and display it in
the category list, but the category index urls will be awesome/ and news/

Fixes #902
(cherry picked from commit 234e92852f397aea67d6e72ea5ca749d58efca96)

Conflicts:
	plugins/category_generator.rb